1. Field of the Invention
The present disclosure relates to an apparatus for estimating a parameter of an induction motor in real time.
2. Background of the Invention
In general, a general-purpose inverter is commonly used to drive 3-phase induction motor, and in particular, a general-purpose inverter is commonly used in a variable speed driving field using an induction motor, a hoisting load, or a traction load of an electric vehicle,
Among parameters of an induction motor, stator resistance and rotor resistance values are changed when a temperature within the induction motor is changed according to a change in a load. Vector control commonly used as an induction motor driving method is significantly dependent upon a motor parameter, so fluctuation of the rotor resistance degrades control performance. This will be described with reference to the accompanying drawings.
FIG. 1 is a view illustrating a configuration of a related art system for controlling an induction motor.
A speed controller 110 receives a rotor speed reference an actual rotor speed and outputs a q-axis current reference in a synchronous reference frame. A current controller 120 outputs d and q-axis voltages in the synchronous reference frame from d and q-axis current references in the synchronous reference frame and an actual current.
A first converting unit 130 converts output voltages from the current controller 120 into voltages in a stationary reference frame, and a second converting unit 140 converts phase currents from an induction motor 160 measured by current sensors 190a, 190b, and 190c into d and q-axis currents in the synchronous reference frame.
An inverter 150 applies voltages to the induction motor 160. A rotor position detecting unit 170 measures a speed of a rotor of the induction motor 160. A magnetic flux angle calculating unit 180 calculates a magnetic flux angle by using the speed of the rotor measured by the rotor position detecting unit 170 and the d and q-axis currents in the synchronous reference frame, and in this case, the d-axis current in the synchronous reference frame may be replaced by a d-axis current reference.
FIG. 2 is a view illustrating a detailed configuration of the speed controller in FIG. 1.
As illustrated in FIG. 2, the speed controller 110 outputs a difference between a reference speed (or speed reference) and an actual speed (or a feedback speed), as a q-axis current reference by using proportional-integral controllers 111a and 111b. A limiter 112 limits an output from the speed controller 110, and a gaining unit 113 provides an anti-windup gain to prevent divergence of the integrator 111b when the limiter 112 operates.
FIGS. 3A and 3B are views illustrating detailed configurations of the current controller in FIG. 1, respectively. Specifically, FIG. 3A is a view illustrating a d-axis current controller in the synchronous reference frame, and FIG. 3B is a view illustrating a configuration of a q-axis current controller in the synchronous reference frame. As illustrated, in order to control d and q-axis currents in the synchronous reference frame, the d and q-axis current controllers include proportional and integral-type controllers 121a and 121b, and 124a and 124b, and feed-forwarding units 122 and 125, respectively.
The feed-forwarding units 122 and 125 may be variously configured according to modeling of an induction motor. When an output from the current controller exceeds a magnitude of a voltage for the inverter to synthesize it, gaining units 123 and 126 provide an anti-windup gain to prevent divergence of integral controllers 121b and 124b. 
An operation of the related art apparatus for controlling an induction motor will be described.
The first converting unit 130 converts voltages in a synchronous reference frame, as outputs from the current controller 120, into voltages in a stationary reference frame, which may be expressed as follows.Vdss=Vdse*cos θe−Vqse*sin θe  [Equation 1]Vqas=Vdse*sin θe+Vqse*cos θe  [Equation 2]
The second converting unit 140 obtains d and q-axis currents in the synchronous reference frame from phase currents of the induction motor 160 measured by the current sensor 190, which may be expressed as follows.
                              i          ds          s                =                                            2              ⁢                                                          ⁢                              i                as                                      -                          i              bs                        -                          i              cs                                3                                    [                  Equation          ⁢                                          ⁢          3                ]                                          i          qs                =                                            i              bs                        -                          i              cs                                            3                                              [                  Equation          ⁢                                          ⁢          4                ]                                          i          ds          e                =                                            i              ds              s                        ⁢            cos            ⁢                                                  ⁢                          θ              e                                +                                    i              qs              s                        ⁢            sin            ⁢                                                  ⁢                          θ              e                                                          [                  Equation          ⁢                                          ⁢          5                ]                                          i          qs          e                =                                            -                              i                ds                s                                      ⁢            sin            ⁢                                                  ⁢                          θ              e                                +                                    i              qs              s                        ⁢            cos            ⁢                                                  ⁢                          θ              e                                                          [                  Equation          ⁢                                          ⁢          6                ]            
The magnetic flux angle calculating unit 180 obtains magnetic flux angles required for angle conversion of the first converting unit 130 and the second converting unit 140, and here, in case of performing indirect vector control, the magnetic flux angles may be obtained as follows.
                              ω          sl                =                                            R              r                                      L              r                                ⁢                                    i              qs              e                                      i              ds                              e                *                                                                        [                  Equation          ⁢                                          ⁢          7                ]                                          ω          e                =                                            P              2                        ⁢                          ω              r                                +                      ω            sl                                              [                  Equation          ⁢                                          ⁢          8                ]                                          θ          e                =                  ∫                                    ω              e                        ⁢                          ⅆ              τ                                                          [                  Equation          ⁢                                          ⁢          9                ]            
Here, ωsl is a slip frequency, Lr is rotor inductance, Rr is rotor resistance, and P is a number of poles.
Meanwhile, in case of performing indirect vector control, rotor resistance is required to obtain a slip frequency by using Equation 7. However, the related art apparatus for controlling an induction motor illustrated in FIG. 1 does not estimate a parameter in real time, which is, thus, vulnerable to a change in a parameter. In particular, a value of rotor resistance of the induction motor 160 is changed according to a change in temperature of the induction motor 160, and a temperature of the induction motor is affected by a change in a load. An error in resistance of a stator occurring in such an environment degrades performance of current control.